In driving stability control, it is, besides other parameters, necessary to know the instantaneous steering angle of the vehicle. As this steering angle depends on the rotational position of the steering wheel it has become usual to determine the steering angle by means of the position of rotation of the steering wheel of a vehicle. To this end, the steering wheel is coupled with a modulator modulating a continuous signal indicative of the rotational position of the steering wheel. This can be done by the modulation of a constant magnetic field or, as well, of a constant light beam. In the present case, a light beam is modulated by the teeth of a code disk. The teeth are arranged in two circular tracks. While the first track as incremental track features a number of teeth evenly distributed over the periphery of the circle and scanned by two sensors staggered by 90 degrees (one tooth period=360 degrees) the so-called absolute track has a chain code distributed over the periphery of a circle and indicating the absolute value of the disk position. Details in this respect have already been described in connection with patent application 195 32 903.1 and thus are not to be explained in further detail.
The background for selecting the two tracks mentioned is that it is possible to determine both the direction of rotation of the steering wheel and the momentary stepwise angle changes in the position of the steering wheel by means of the incremental track by way of the two sensors. On the other hand, these data do not enable the determination of the initial angle position of the steering wheel upon the turning-on of measurement such as upon the start of the vehicle since, in this case, it is possible to define but relative changes only if the electric power supply necessary for the steering angle determination has been switched on and if, at least, a small change in the angle of rotation of the steering wheel has taken place.
In the abovementioned patent application a resolution of 1.5 degrees was suggested with regard to the absolute value which corresponds to a tooth number of 60 teeth on the incremental track. 240 steps were defined with regard to the step number of the absolute value code. Thus, there results an initialisation angle of 12 degrees with a total of three sensors.
So as to be able to define, at least, 240 different values on the absolute value track an 8-bit code is nessecary which allows the definition of 256 steps at the maximum. The manufacture of such a big step number in a comparatively small code disk is only possible by selecting comparatively high tolerances and is thus quite difficult and expensive.
It is an object of this invention to reduce the number of bits on the absolute track that are needed for the step number of a code.
This object is achieved by the present invention by using the code track and also the momentary position of the incremental track, relative to the two sensors scanning this track, in order to determine the absolute value of the steering wheel angle. In this way, it is possible to obtain at least 1 bit of the needed 8 bit from the incremental track. Advantageous further embodiments show that, additionally, it is even possible to obtain a second bit from the state of the incremental track so that it is possible to cut down on 2 bit on the absolute track, the evaluation of 6 bit being sufficient there. Thus, it will be sufficient to code 60 values on the absolute track instead of 240 values.
Further above, it was already mentioned that at least 1 bit of coding of the absolute track can be saved by applying the present invention. Thence it follows that, with the resolution being the same, the number of the values exclusively determinable by the absolute track is halved.
So as to be also able to recognise the direction of rotation of the steering wheel, the incremental track is scanned by two sensors. According to a preferred embodiment, it is expedient to define the actual absolute steering angle, i.e. the absolute value determinable by means of the incremental track and the absolute track, by 8 bit whereby it will be no problem to express 240 different values which corresponds to a resolution of 1.5 degrees. Such a resolution is quite sufficient for the at present current driving stability control systems. This invention, however, will also be expedient in cases where a higher resolution is desired.
The present invention also allows the absolute value to be defined by six bits as it relates to the absolute track and to take the additional required 2 bit from the two output signals of the incremental sensors. As the six-digit code on the absolute value track is to be a chain code it is possible to get along with approximately 70 steps on the absolute track which considerably simplifies the manufacture of the code disk.
Using a chain code puts strong restrictions on selecting the code values on the absolute track since, each time, six consecutively read bit are not to be repeated on the track. Further, the present invention teaches the assignment of stepwise changing absolute angle values to these abruptly changing code values.